The present invention relates to the preparation of chlorine substituted chloroformates. More particularly, the invention relates to preparing such compounds in a process that is continuous and has high yields and purity. The process is described with particular reference to the preparation of 2-chloroethyl, 2-chloroisopropyl, 3-allyloxy 1-chloroisopropyl, and 3-phenoxy 1-chloroisopropyl chloromates. While it is not believed to be so limited, certain reservations are expressed with respect to across-the-board application to all chlorine-substituted compounds of the same type.
These compounds are useful in the preparation of flotation reagents of the xanthogen formate type, and in other organic syntheses. It should be noted at the outset that both reactants and products are both toxic and corrosive, and must be handled and used with appropriate precautions.
Chloroformates are the reaction product of phosgene (carbonyl chloride) and organic alcohols, oxides, and some carbonyls. Insofar as known, the reactions are always highly exothermic, and are plaqued by low yields (with a variety of by-products) requiring elaborate purification procedures. Most prior art teachings are restricted to one or a small group of compounds, and this is in accord with my own and coworker's experience, in that what works for one set of reactants will not necessarily work with even a closely related set. An exception to this is the patent of Strain et al., U.S. Pat. No. 2,476,637. There, reaction under conditions of total reflux is disclosed, and while this is believed to be appropriate for the specific system disclosed, it would be very inadequate for many of the long list of proposed substitute reactants. The following patents are considered more limited and, hence, more typical.
U.S. Pat. No. 2,820,809 and 2,820,810 of Frevel et al. disclose manufacture of 2-chloroethyl chloroformate and 2-chloro-1-methylethyl chloroformate, respectively. In the first case, reactants are in the gaseous state, phosgene being reacted with ethylene oxide in the presence of HCl vapor as catalyst. In the second case, liquids are used at temperatures near 0.degree. C., propylene oxide and phosgene are reacted, again with added HCl. In both cases product is purified by distillation, and yield is about 50-55%. Of course, the need to continuously add the catalyst, plus removal of same, insofar as possible, from the product, is a significant cost factor.
More recently, in U.S. Pat. No. 4,039,569 Bell et al., assigned to the same assignee as the instant application, a continuous process of making methyl chloroformate by reacting liquid methanol with phosgene in a large, circulating load of pre-formed chloroformate, at 15.degree.-16.degree. C. is disclosed. Product at 98% pure, and yields over 80% are reported. Two points are of interest with this process: First, it is carried out without any catalyst at all. Second, and perhaps more important is exemplifying the state of this art, the same general process could not be employed to produce ethyl chloroformate with any degree of success.
The stoichiometry of the reaction between phosgene and epoxides has been studied to a limited extent (Jones, J. Chem. Soc. 1957, 2735-43, Chem. Ab. 51, 16433b). All reactions were carried out at 10.degree. C. with pyridine present, over 1,25 hours, followed by distillation. Generally, it was determined that chloroformates resulted from 1:1 molar proportions, but chloroalkyl carbonates resulted if the epoxide quantity was doubled. Yield was apparently about 80%, but purity was not reported. Fifty percent yields with ethylene oxide and propylene oxide were reported earlier by Malinovskii et al. with ethylene bromide as solvent, at 0.degree. C., (Chem. Ab. 48, 2580C).
It is at least possible that some confusion has resulted from prior workers calling true epoxy or epoxide compounds by the more generic "oxide". As used herein, epoxide is intended to mean the true epoxy structure, wherein the oxygen is bound to two separate atoms that are otherwise joined.